team: fix checks in team_get_first_port_txable_rcu()

should be checked if "cur" is txable, not "port".

Introduced by commit 6e88e135 "team: use function team_port_txable()
for determing enabled and up port"

Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>